Low-angle Peak Research Articles

Structural and electrical properties of the α-form of metal-free phthalocyanine (α-H2Pc) semiconducting thin films

Abstract An X-ray structural study of thermally evaporated metal-free phthalocyanine thin films with various film thicknesses was performed. All samples studied had polycrystalline structure and the unit cell was found to be of the α-form. Variation of the deposition rate from 0.5 to 1 nm s −1 had little effect on the structure. The films exhibit preferential orientation at low thickness; however, at higher thickness they become less orientated as additional peaks appear in the spectrum. The increase in the intensity of the first significant low angle peak with increasing thickness is attributed to the increased volume of the crystal probed during the X-ray exposure. The current density–voltage ( J – V ) characteristics of α-H 2 Pc films sandwiched between two aluminum electrodes showed ohmic behavior at low voltages and space–charge-limited conduction (SCLC) at higher voltages. For comparison, similar measurements of the current density as a function of voltage were performed on zinc phthalocyanine, ZnPc, thin films using aluminum electrodes. The J – V characteristics showed ohmic behavior at low voltages followed by SCLC dominated by an exponential trap distribution at higher voltages. Consequently, in both H 2 Pc and ZnPc films, aluminum electrodes act as if they are ohmic contacts. The implied provision of ohmic contacts using aluminum in this case is attributed to the formation of a thin Al 2 O 3 layer during the deposition process.

Preparation and Electrorheological Activity of Mesoporous Rare-Earth-Doped TiO2

A new kind of mesoporous rare-earth-doped TiO2 particle with a well-defined crystalline framework was synthesized under a general temperature state for use as electrorheological (ER) active material. The neutral surfactant dodecylamine (DDA) was used as a template to direct the mesoporous structure. The low-angle X-ray diffraction peak showed that the mesoporous rare-earth-doped TiO2 had an interlayer distance of about 3.4 nm and the high-angle peaks showed that the material possessed an anatase crystalline framework. ER properties of the suspension based on this material and silicone oil were measured in a dc electric field. It had been shown that the extraordinary high yield stress over 8.1 kPa could be induced when a 3 kV/mm electric field was applied. It was 20 times higher than that of pure TiO2 ER suspension and twice as high as that of single-doped TiO2 ER suspension as reported in our previous works. Interestingly, the shear stress of this suspension was found to continuously increase with temperature elevation in the range 10−100 °C. These improvements were attributed to the pursuit of slow polarization and suitable conduction properties of this mesoporous-doped TiO2 suspension in terms of the measured results of dielectric and conduction properties. The present experimental result suggests that both the active internal structure and interface or surface structure of ER particles, which merit the pursuit of suitable dielectric and conduction properties, are the key to high ER activity.

MCM-69, a novel layered analogue of EU-19

A new form of crystalline silica has been prepared and is designated MCM-69. The material has a crystalline layered structure which can be delaminated and which contains a regular array of reactive, surface silanols. Specifically, one out of every three Si atoms in the MCM-69 structure contains a silanol group. To maintain layer separation, considerable care must be taken in the preparation of MCM-69 to avoid dehydration of these silanols and inadvertent crosslinking of the silica layers, which would prevent subsequent delamination. Stoichiometrically, MCM-69 has the idealized formula, H 2O·6SiO 2, and is derived from MCM-69(P), a piperazine (pipz) silicate precursor. MCM-69(P) appears to have the same topology as EU-19. MCM-69(P) differs from EU-19 in its behavior, most notably in the removal of its pipz to yield MCM-69, and its subsequent dispersability. MCM-69 can be swollen and dispersed in aqueous solution by reacting the silanol groups with amines, such as pyrrolidine, n-propylamine, or n-octylamine. The resultant products all show crystallinity as characterized by a strong X-ray diffraction (XRD) peak below 10.5° 2 θ. The exact position of the low-angle peak indicates the size of the amine controls the interlayer separation. The larger the amine, the lower the angle of the XRD peak, and the larger the separation between the silica hydrate layers. The silanol groups react with hydroxide ion and with surface functionalizing agents such as hexamethyldisilazane.

Erosion of in-situ TiC particle reinforced Al–5Cu composite

Erosion behavior of in-situ 15 vol% TiC particle reinforced Al–5Cu based composite and the monolithic Al–5Cu alloy was studied at oblique and normal impacts. The erosion rate of the composite was higher than that of the monolithic alloy. There existed low-angle peaks in erosion rate vs. impact angle curves for the two materials, which indicated ductile erosion behavior although the composite had a low tensile ductility. Theoretical model for ductile materials cannot predict the erosion rate of the composite. The presence of TiC particles in the composite increased, rather than decreased the erosion rate even though the particles obviously increased the hardness of the composite. Erosion process involved the displacement of the target material and the removal of the displaced material. By comparison to the monolithic alloy, the composite exhibited higher resistance to the displacement of the target material due to higher hardness, while lower resistance to the removal of the displaced material due to poor ductility. The combined effect from these two aspects resulted in a higher erosion rate of the composite than that of the monolithic alloy.

Intrafibrillar mineral may be absent in dentinogenesis imperfecta type II (DI-II).

High-resolution synchrotron radiation computed tomography (SRCT) and small-angle x-ray scattering (SAXS) were performed on normal and dentinogenesis imperfecta type II (DI-II) teeth. The SRCT showed that the mineral concentration was 33% lower on average in the DI-II dentin with respect to normal dentin. The SAXS spectra from normal dentin exhibited low-angle diffraction peaks at harmonics of 67.6 nm, consistent with nucleation and growth of the apatite phase within gaps in the collagen fibrils (intrafibrillar mineralization). In contrast, the low-angle peaks were almost non-existent in the DI-II dentin. Crystallite thickness was independent of location in both DI-II and normal dentin, although the crystallites were significantly thicker in DI-II dentin (6.8 nm [SD = 0.5] vs. 5.1 nm [SD = 0.6]). The shape factor of the crystallites, as determined by SAXS, showed a continuous progression in normal dentin from roughly one-dimensional (needle-like) near the pulp to two-dimensional (plate-like) near the dentin-enamel junction. The crystallites in DI-II dentin, on the other hand, remained needle-like throughout. The above observations are consistent with an absence of intrafibrillar mineral in DI-II dentin.

X-ray diffraction evidence for the formation of a discotic phase during graphitization

The low-angle peak with a spacing of 10 Å observed in X-ray diffraction scans of partially pyrolyzed aromatic molecules is attributed to the formation of a liquid crystalline pre-graphitic phase that we identify as discotic. This conclusion is substantiated using data obtained for partially pyrolyzed 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). The crystalline reflections of PTCDA are replaced by amorphous scattering during pyrolysis at 530°C. In addition to the commonly observed amorphous carbon peak providing an interlayer spacing of about 3.5 Å, a low-angle peak corresponding to a 10 Å spacing is observed in partially pyrolyzed PTCDA. Based on simulation of structure and diffraction patterns using molecular dynamic calculations, this peak is attributed to the distance between columns of rotationally disordered, partially pyrolyzed PTCDA molecules in a discotic phase.

Synthesis and characterization of tubular aluminophosphate mesoporous materials containing framework magnesium

A series of aluminophosphates (and magnesium-aluminophosphates based on tubular molecular sieves have been synthesized using cetyltrimethylammonium bromide (CTAB) as structure-directing agent in the presence of tetramethylammonium hydroxide, at room temperature and under hydrothermal synthesis, using molar ratios of CTAB/Al=0.5 and 1.0, and 0⩽z (=MgO/Al2O3)⩽0.015 in the gel composition. The hexagonal mesostructure of the as-synthesized aluminophosphates and magnesium-aluminophosphates was evidenced by two defined diffractions in the range between 2θ=2° and 5°, and was obtained regardless of the amount of magnesium added to the synthesis mixture. The usual processes for removal of structure-directing agent, i.e. calcination and Soxhlet extraction with azeotropic solutions, both resulted in framework collapse, possibly due to the poor polymerization of the inorganic matrix. An alternative procedure was performed by extracting CTAB using a solution of ethanol/n-butylamine, which showed to be efficient in extracting the majority of the organic part, without destruction of the mesostructure. After extraction, the samples exhibited only the characteristic low angle peak, typical of tubular materials, whose structure remained after further calcination. Elemental analyses confirmed the presence of magnesium in these materials. The 31P nuclear magnetic resonance spectra of calcined sample confirmed the better condensation of the sample after thermal treatment, compared to the as-synthesized materials. The transmission electron microscopy image showed disordered arrays of tubes. These results indicates that thermally stable mesoporous magnesium-aluminophosphate materials can be obtained by using alkaline extraction and calcination as post-synthesis treatment.

ON THE CHARACTERIZATION OF METALLIC SUPERLATTICE STRUCTURES BY X-RAY DIFFRACTION

To solve the problem on the microstructural characterization of metallic superlattices, taking the NiFe/Cu superlattices as example, we show that the structures of metallic superlattices can be characterized exactly by combining low-angle X-ray diffraction with high-angle X-ray diffraction. First, we determine exactly the total film thickness by a straightforward and precise method based on a modified Bragg law from the subsidiary maxima around the low-angle X-ray diffraction peak. Then, by combining with the simulation of high-angle X-ray diffraction, we obtain the structural parameters such as the superlattice period, the sublayer and buffer thickness. This characterization procedure is also applicable to other types of metallic superlattices.

Scaling of Small-Angle Neutron Scattering Intensities from Gelling Colloidal Silica

Small-angle neutron scattering data, taken as a function of time from initiation of gelation in colloidal silica suspensions, with silica mass fractions ranging from 15 to 30%, are presented. Over a wide range of initial pH, the measured structure factor S(q) contains a low angle peak that, as time progresses, grows in height and moves to lower wave vectors q. Sometime after the gels have set, this peak stops growing, marking the end of the reaction. The data scale according to the relation, S(q,t)~\(q_m^{-d_f}\)(t)(q/qm(t))\(\tilde S\), where qm(t) is the wave-vector location of the low angle peak as a function of time t from initiation, df is a fractal dimension, and \(\tilde S\) is a characteristic structure function. The exponent df is insensitive to the silica mass fraction but that the form of \(\tilde S\) is mass fraction dependent.

Effect of local sequence inversions on the crystalline antiparallel β-sheet lamellar structures of periodic polypeptides: implications for chain-folding

The crystal structure and texture of the monodisperse periodic polypeptide [(AG) 3EG(GA) 3EG] 10 (poly(±AG) 3EG; A=alanine, G=glycine, E=glutamic acid) were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, and electron microscopy. Structure determination was aided by comparison with the recently described structure for the related periodic polypeptide [(AG) 3EG] 36 by Krejchi et al. (Macromolecules 1997;30:5012). Texture-oriented samples of poly(±AG) 3EG were obtained by crystallization of the polymer from aqueous formic acid solution. The evidence supports an antiparallel (ap) β-sheet protein structure and the X-ray diffraction signals index on an orthorhombic unit cell with parameters: a=0.950 nm (hydrogen-bond direction), b=1.052 nm (ap β-sheet stacking direction), c=6.95 nm (chain direction). The absence of the (010) diffraction signal, a prominent signal in the poly(AG) 3EG diffraction pattern, implies that the ap β-sheets are `apolar', i.e. both surfaces are equally populated with alanyl methyl groups. Selective line broadening of wide-angle diffraction signals with ℓ≠0 gives an estimated crystal size of ≅4 nm in the chain direction. This observation, coupled with the appearance of low-angle particle interference peaks, indicates a crystal thickness considerably less than the chain length and suggests an adjacent-re-entry chain-folded lamellar structure incorporating the ap β-sheet architecture. The polypeptide folds through γ-turns, in-phase with the pseudo-octapeptide repeat; the glutamic acid residues occur on the lamellar surfaces. These results and those from the crystalline lamellae of poly(AG) 3EG suggest that β-turns are not compatible with these repetitively stacked ap β-sheet structures. This implies that intersheet interactions of alanyl methyl groups and glycyl α-protons are not sufficiently strong to dictate the folding geometry in these structures.

SWELLING OF ASPHALTENES

ABSTRACT We report x-ray scattering measurements on asphaltenes and bitumen solutions, with attention to the concentrated regimes. We characterize the data in terms of a correlation length which is indicative of proximity to phase separation in a mixture, rather than the radius-of-gyration more indicative of individual colloids. By studying the behavior of the low-angle asphaltene peak on dilution, we show that asphaltenes swell. These results allow the drawing of a phase diagram with two regimes separated by a critical point: a colloidal regime, and a“ swollen asphaltene” regime, the latter being much larger.

Molecular packing of type I collagen in tendon

X-ray diffraction of rat tail tendon shows that type I collagen fibrils contain regions of three-dimensional crystalline arrays; where molecular packing is speculated to be by a staggered sheet or microfibril arrangement. The X-ray diffraction pattern also contains a significant amount of diffuse scatter indicative of static and thermal disorder in fibrils. Removal of the diffuse scatter from the equatorial region of X-ray diffraction patterns obtained using synchrotron radiation allowed the Bragg intensities to be viewed on a flat background. Indexing of Bragg peak intensity on the 10, −10, 0 –1, 01, −11 and 1 –1 row-lines of the triclinic unit cell have been used here to test possible sheet and microfibril packing arrangements. The relative translation of molecular segments in the gap and overlap regions as well as the telopeptide orientation have been investigated. A global search through combinations of molecular packing and molecular translation revealed that the sheet-type conformations cannot account for the observed low-angle off-meridional Bragg peak intensity distribution. A superior fit is obtained with D-staggered left-handed microfibril structures. The orientation of the telopeptides may indicate that there are interconnections between microfibrils that may explain the difficulty in isolating individual microfibrillar structures.

Crystal Structures of Chain-Folded Antiparallel β-Sheet Assemblies from Sequence-Designed Periodic Polypeptides

The crystal structures and textures of a family of sequence-designed periodic polypeptides were investigated and analyzed using X-ray diffraction, vibrational spectroscopy, and cross-polarization magic angle spinning 13C nuclear magnetic resonance. The repetitive amino acid sequences are described by −[(AG)xEG]−, with integer x from 3 to 6. These macromolecules were prepared via bacterial expression of artificial genes and are monodisperse. Crystalline samples were obtained, and the interpretation of the X-ray diffraction results was aided by the generation of computer-simulated X-ray diffraction patterns. This allowed direct comparisons to be made with the observed texture-oriented X-ray diffraction photographs. All diffraction and spectroscopic evidence supports an antiparallel (ap) β-sheet structure, and all structures index on orthorhombic sublattices similar to those reported for Bombyx mori silk fibroin and poly(l-alanylglycine). The unit cell parameters for poly(AG)3EG, for example, are a = 0.948 nm (hydrogen-bond direction), b = 1.060 nm (ap β-sheet stacking direction), and c = 0.695 nm (chain direction). Selective line broadening is observed for wide-angle diffraction signals with l ≠ 0 (for the 211 in particular) and gives an estimated crystal size of <4 nm in the chain direction. This, coupled with the appearance of a low-angle particle interference peak at 3.6 nm, indicates a crystal size over an order of magnitude less than the chain length and suggests an adjacent reentry chain-folded lamellar structure incorporating the ap β-sheet architecture. A structure with polar ap β-sheets and γ-turns, stacking with the hydrophobic methyl groups of the alanyl residues in contact, is selected by X-ray structure refinement to give the best match with the experimental data. The pattern of crystallization behavior of the poly(AG)xEG family is consistent with the folding periodicity being in-phase with the amino acid sequence so that the glutamic acid residues are confined to the lamellar surfaces.

Preparation of BaTiO3Thin Films Using Glycolate Precursor

The translucent grease-like gel was obtained by heating the mixed solution of monohydrated barium hydroxide and titanium isopropoxide in ethylene glycol up to 145°C. The gel consisted of EG (ca. 90%) and residual flake-like microcrystals with a characteristic low-angle X-ray diffraction peak (d=1.187 nm). The thermal decomposition of the gel proceeded through three major stages, viz., (i) evolution of EG (∼200°C), (ii) decomposition of EG moiety in microcrystal to form an amorphous carbonate (∼400°C), and (iii) crystallization of cubic BaTiO3with a grain size of 10 to 20 nm (∼600°C). Spectroscopic and thermoanalytical techniques were used to characterize the glycolate precursor and the intermediate isolated at various stages. Smooth and transparent BaTiO3films with 2–3 μm thickness were obtained by painting the gel once on several substrates and heating up to above 600°C. BaTiO3films prepared were strongly oriented along the (110) plane.

Ionization in coplanar symmetric (e,2e) experiments of N2 and CO at intermediate energies.

Measurements of the triple differential cross sections in coplanar symmetric energy sharing geometry for the two isoelectronic molecules ${\mathrm{N}}_{2}$ and CO are presented between 90 and 400 eV incident energy. At 400 eV, our results are compared to those of previous (e,2e) experiments done in other geometries and to photoionization data. At this incident energy, a comparison is also made with the plane wave impulse approximation (PWIA). Our calculations show that this does not perfectly describe the measured cross sections. Measurements made over an extended angular range, 30\ifmmode^\circ\else\textdegree\fi{}--120\ifmmode^\circ\else\textdegree\fi{}, present two maxima: that at large angle results from backscattering of the projectile prior to ionization of the target and is not explained by the PWIA. As energy is decreased, as for atoms, the relative importance of the low angle peak diminishes and that of the large angle peak increases: we then lack an adequate theoretical model and only comparison with data obtained for atomic targets under similar kinematical conditions is possible. Our high energy resolution permits the observation of several satellite structures for both target molecules, in particular for CO. At 400 eV, the assignment of these to different residual ion primary hole states correlates with the forms of the angular distributions. \textcopyright{} 1996 The American Physical Society.

Dynamic structure factor scaling in dense gelling silica suspensions

Small-angle neutron scattering data from dense (30% by mass) gelling 7 nm colloidal silica spheres are presented. The coarsening process that occurs during gelation exhibits temporal self-similarity, and the time-dependent structure factor obeys the dynamic scaling relation . Here, q is the scattered wavevector, is the location of the low-angle peak in is a time-independent characteristic structure function which has a maximum at x = 1, and is the fractal dimension. Connections between the silica gelation and spinodal decomposition in a simple fluid are reviewed.

Structure of gel phase saturated lecithin bilayers: temperature and chain length dependence

Systematic low-angle and wide-angle x-ray scattering studies have been performed on fully hydrated unoriented multilamamellar vesicles of saturated lecithins with even chain lengths N = 16, 18, 20, 22, and 24 as a function of temperature T in the normal gel (L beta') phase. For all N, the area per chain Ac increases linearly with T with an average slope dAc/dT = 0.027 A2/degree C, and the lamellar D-spacings also increase linearly with an average slope dD/dT = 0.040 A/degree C. At the same T, longer chain length lecithins have more densely packed chains, i.e., smaller Ac's, than shorter chain lengths. The chain packing of longer chain lengths is found to be more distorted from hexagonal packing than that of smaller N, and the distortion epsilon of all N approaches the same value at the respective transition temperatures. The thermal volume expansion of these lipids is accounted for by the expansion in the hydrocarbon chain region. Electron density profiles are constructed using four orders of low-angle lamellar peaks. These show that most of the increase in D with increasing T is due to thickening of the bilayers that is consistent with a decrease in tilt angle theta and with little change in water spacing with either T or N. Because of the opposing effects of temperature on area per chain Ac and tilt angle 0, the area expansivity alpha A is quite small. A qualitative theoretical model based on competing head and chain interactions accounts for our results.

Aggregation of a quenched Lennard-Jones system under shear.

The thermodynamic decomposition of an unstable thermostatted system of Lennard-Jones disks is investigated by nonequilibrium molecular dynamics. The system, first unsheared and then subjected to planar Couette flow, is studied after temperature quenches into the unstable vapor-liquid and the vapor-solid coexistence regions of the phase diagram. An interconnected morphology, characteristic of spinodal decomposition, forms after quenching. The cluster growth is found to be temporally self-similar, and the structure factor S(q,t) obeys the dynamic scaling relation S(q,t)\ensuremath{\sim}${\mathit{q}}_{\mathit{m}}^{\mathrm{\ensuremath{-}}{\mathit{d}}_{\mathit{f}}}$(t)S\ifmmode \tilde{}\else \~{}\fi{}[q/${\mathit{q}}_{\mathit{m}}$(t)]. Here, q is the scattered wave vector magnitude, ${\mathit{q}}_{\mathit{m}}$(t) is the location of the low angle peak in S(q,t), S\ifmmode \tilde{}\else \~{}\fi{}(x) is a time-independent structure function which has a maximum at x=1, and ${\mathit{d}}_{\mathit{f}}$ is a fractal dimension. ${\mathit{d}}_{\mathit{f}}$ is relatively insensitive to the postquench state point, but may depend on the shear rate. The primary influence of shear is to accelerate the aggregation---an effect that has also been observed experimentally in dense gelling silica suspensions. The similarities between these simulations and experiment suggest that a characteristic fractal dimension of a dense gel may be determined from measurements of S(q,t). \textcopyright{} 1996 The American Physical Society.

Preparation of EuO and EuO/Cu multilayered films and their magnetic properties

Europium monoxide (EuO) simple films (ESFs) and EuO/Cu multilayered films (EMFs) have been prepared successfully by a molecular beam epitaxy method using an oxygen atmosphere. The structural and magnetic properties depend highly on the source material (metal Eu or Eu 2O 3) and the oxygen atmospheric pressure. Nearly epitaxial growth was observed for ESF. Metal-insulator transitions were observed for some ESFs. Though epitaxial growth was not achieved, the formation of superstructures in some EMFs was confirmed by low-angle X-ray diffraction peaks. The magnetization measurements for EMFs indicate approximately the same saturation magnetization M s and Curie temperature T c as the bulk values, even for an EMF with EuO layers as thin as 2.5 nm.

Microstructure and magnetic anisotropy of ultrathin Co/Pt multilayers grown on GaAs (1̄1̄1̄) by molecular-beam epitaxy

Multilayers of (Co3 Å, Pt15 Å)x, x=15 or 30 repeats, with or without a 200 Å silver buffer layer, were grown on GaAs (111) substrates by molecular-beam epitaxy. Vibrating sample magnetometry measurements confirmed that the samples with the Ag buffer layer show strong uniaxial magnetic anisotropy perpendicular to the surface. The perpendicular anisotropy exhibited by these metallic superlattices is discussed in terms of the microstructure of the overall multilayer stack, as well as the structural characteristics of the Co interface layer. Samples grown on the Ag buffer layer show strong (111) texture with 30–40-nm-size twin-related grains. These grains, correspond to the two possible (111) stacking sequence for an fcc lattice, i.e., double positioning. However, direct growth on GaAs (111) results in randomly oriented 10–20 nm grains. All samples exhibit a repeat period of 1.83 nm in both low-angle reflectivity and high-angle Θ–2Θ x-ray scattering measurements. In addition, transverse scans through the low-angle multilayer Bragg peaks show the interfaces to be diffuse in nature indicative of considerable in-plane inhomogeneity and/or compound formation. High-resolution electron microscopy measurements of cross sections compared with image simulations confirm that the interface layer is diffuse and its stoichiometry is such that the Co occupation is less than 40%. Redistribution of Co should then extend over at least four monolayers. The nanostructure of the samples grown with the Ag buffer layer comprises an eight atomic layer repeat with the Co interface layer diffuse over four monolayers. The microstructure is strongly (111) textured with columns of twin related 30-nm-sized grains. It is suggested that the combination of interdiffusion, highly oriented but twin-related columnar growth, small grain size with a possible nanometer-scale second phase may be the key to the understanding of the perpendicular anisotropy observed in these (111) superlattices.